[ARM] Wire up the new scheduler description for the ARM Cortex-A57 processor

[ARM] Wire up the new scheduler description for the ARM Cortex-A57  processor

	* config/arm/arm-cores.def (cortex-a57): Use the new Cortex-A57
	pipeline model.
	config/arm/arm.md: Include the new Cortex-A57 model.
	(generic_sched): Don't use generic_sched when tuning for
	Cortex-A57.

From-SVN: r220103